This invention relates to a magnetron drive power supply with a magnetron of a microwave oven, etc., as a load.
A magnetron drive power supply in a related art will be discussed with reference to the accompanying drawings (FIGS. 8 and 9). FIG. 8 is a circuit block diagram of a magnetron drive power supply in a related art. A semiconductor switch in a high-frequency inverter 2 is controlled by controller 7, whereby a commercial power supply 1 is converted into radio-frequency power of 20 to 50 kHz and the radio-frequency power is supplied to a high-voltage transformer 3. A high-voltage rectification circuit 4 and a magnetron 5 are connected to the secondary side of the high-voltage transformer 3 and a DC high voltage is applied to the magnetron 5 for generating a 2.45-GHz radio wave.
Zero-voltage detector 6 detects a zero voltage point of the power supply voltage 1 and causes modulation signal generator 9 to generate a modulation waveform responsive to the power supply phase. Upon reception of input of zero voltage detection from the zero-voltage detector 6, the modulation signal generator 9 outputs a modulation waveform of one period of the power supply voltage 1 as a peak value responsive to the setup value of input current. Using such a modulation signal, the controller 7 can control the input current to the form close to a sine wave. The controller 7 performs 20 to 50-kHz PWM modulation of the modulation signal by oscillator 10 and transmits the signal to driver 8, thereby controlling the on-duration of the semiconductor switch in the high-frequency inverter 2. As the zero-voltage detector 6, voltage detection with a transformer using a photocoupler, etc. is available. And, as the controller 7, control of a microcomputer, etc., is used.
FIGS. 9A to 9D are waveform charts of the magnetron drive power supply in the related art. Upon reception of a signal of commercial power supply (FIG. 9A), a signal of zero voltage detection (FIG. 9B) oscillated at the timing of zero voltage is output by the zero-voltage detector 6. The rising edge of the signal of the zero-voltage detector 6 is detected and a modulation signal (FIG. 9C) preset so that the input current becomes a predetermined value and moreover the power factor of the input current becomes close to 1 is output for one period of the commercial power supply 1. The modulation signal (FIG. 9C) is compared with the oscillation frequency of oscillator output (FIG. 9D) by comparator 11, whereby the signal is subjected to PWM modulation and is supplied to the driver 8 as a drive signal. The modulation signal is set so that the frequency of the semiconductor switch in the high-frequency inverter 2 becomes 20 to 50 kHz. The controller 7 performs such control, whereby electric power having a current waveform with a less harmonic component with a good power factor can be supplied.
However, in the magnetron drive power supply in the related art, if the zero voltage detection shifts due to noise, instantaneous power interruption, etc., the modulation waveform deviates from the essential timing and a possibility of leading to a failure of the high-frequency inverter because of overvoltage, overcurrent, etc., occurs.
It is therefore an object of the invention to provide a magnetron drive power supply which is resistant to change in the power supply environment and can operate stably.
According to the first aspect of the invention, there is provided a magnetron drive power supply comprising: a commercial power supply; a high-frequency inverter which converts electric power of the commercial power supply into high-frequency power and supplies the high-frequency power to a high-voltage transformer; a high-voltage rectification circuit and a magnetron being connected to secondary output of the high-voltage transformer; zero-voltage detector which detects zero voltage of the commercial power supply; and controller which controls the high-frequency inverter in response to output of the zero-voltage detector, wherein the controller predicts the detection timing of zero voltage by the zero-voltage detector in each period and enables the output from the zero-voltage detector to be received only for a given time before and after the predicted detection timing.
Thus, even if the zero-voltage detector or the power supply voltage carries noise, the voltage zero point is not largely mistaken, so that overcurrent, overvoltage, etc., does not occur and the magnetron drive power supply that can stably operate can be realized.
Preferably, if the given time before and after the predicted detection timing contains a period in which the output from the zero-voltage detector is not received, it is assumed that the output from the zero-voltage detector is received, and controlling the high-frequency inverter is continued.
Thus, it is made possible to continue the operation with safety if short-time instantaneous power interruption of the commercial power supply occurs, and the magnetron drive power supply that can stably operate without stopping an inverter unnecessarily can be realized.
Preferably, if a period in which the output from the zero-voltage detector is not received occurs successively a stipulated number of times in the given time before and after the predicted detection timing, the controller stops the high-frequency inverter.
Thus, it is made possible to stop the inverter with safety if comparatively long-time instantaneous power interruption of the commercial power supply occurs, and the magnetron drive power supply that can operate without a failure caused by a power outage can be realized.
According to the second aspect of the invention, there is provided a magnetron drive power supply comprising: a commercial power supply; a high-frequency inverter which converts electric power of the commercial power supply into high-frequency power and supplies the high-frequency power to a high-voltage transformer; a high-voltage rectification circuit and a magnetron being connected to secondary output of the high-voltage transformer; input current detector which detects the current value of the high-frequency inverter; and controller which controls the high-frequency inverter, wherein if the detection value of the input current detector has a predetermined difference from a target value continuously for a given time, the controller stops the high-frequency inverter.
Thus, it is made possible to detect the power supply voltage lowering without detecting the input voltage, and the magnetron drive power supply having the voltage lowering protection function can be realized at low cost.
Preferably, the predetermined difference between the detection value of the input current detector and the target value is set in response to the target value.
Thus, it is made possible to detect the input voltage of the commercial power supply lowering almost at constant voltage independently of the input current, and the magnetron drive power supply having the voltage lowering protection function can be realized at low cost.